In 1618-1648 Slupsk experienced great demographic fall as the result of the Thirty-Year War. The city experienced a big decrease in habitancy through 100 years. In the final phase of this war Slupsk counted only 2300 inhabitants. In 1756, when next great war-conflagration approached, the Seven Years' War (1756-1763), city counted approx. 4500 people so, as many as before the outbreak of Thirty Years' War. Losses among the civil population during the Seven Years' War were about 1000 people. Before Slupsk city contrived to rebuild its own potential of population, another disaster affected the city 1744-1745. It was a plague which absorbed about 500 people, mainly women and children. At the end of 18th century the average number of Slupsk inhabitants oscillated about 4700 people. The national structure of population of Slupsk region changed fundamentally after 1720.

The more and more numerous and more efficient magistrate apparatus executing Prussian reason of state, and also widely guided colonization action, brought effects in increase of people quantity, declaring German nationality. Only in Koszalin district (so and Slupsk ground) in 1762-1786 3774 farms for sixteen thousands settlers brought from all lands of Reich were created. The effect was that about 43 new villages appeared on the map. The fraction of Slovinian population inhabiting grounds of Gardno and Lebsko lakes until the end of 19th century kept its national identity and separateness. Despite of efficiently working mechanisms of Prussian state legislation, this did not mean the absolute elimination of Slav national element in Slupsk and its nearest area.

The parish of St. Peter in the Old City embraced 16 villages in which 70% of population used the Kashubian dialect. Deacons of this church on turn 17th and 18th century not only celebrated church services in Polish language but taught catechism and religious songs in Kashubian dialect. In 1628 in the parahospital Chapel of St. Spirit, lying outside precincts of urban walls of the New Gate church services were held in Polish language. After the abolition of these services the population of Pomeranian-Kashubian origin (settled on the New City) used service of deacon at Church of St. Peter. The town council in 1722, despite of unfavorable position of provost of Slupsk synod, supported affairs of 'polish' parish nobility, declaring that "...nobody to be a deacon on Old City cannot be appointed if does not understand the Polish language...".

In 18th century Slupsk population grew up to about 60%. Probably, the majority of this increase supplied the New City. The urban part of the Old City in the half of 18th century counted about 40 residential buildings. About 15 buildings were located on suburbs, in front of the New Gate, toward the tract till Slawno. Region of the New City beside Holstein Gates was intended under housing which in this area was embraced with profitable tax discounts. The Smith's Gate, Town Hall, Latin School, fragments of city walls and other buildings of public utility needed some renovations. Ruined medieval downtown complex was replaced with three-storey compact urban tenements with differentiated rooftops and different sculpture of facades. Unfortunately, a giant part of urban building complex from this period was damaged by Red Army in 1945 . One of few remainders of former habitable architecture at Old Market is townsfolk tenement and so- called Courtly Pharmacy from about 1783.